This invention relates to sound-absorbing acoustic ducts, and more particularly to acoustic ducts with peripherally semented or peripherally and longitudinally segmented treatments for absorbing spinning mode acoustical energy. As a typical application, the invention relates to noise suppressing linings for the inlet of an aircraft jet engine fan.
The sound pressure field generated by an axial flow compressor, commonly referred to as a fan, contains rotating pressure patterns or spinning modes caused primarily by interaction of the rotor and stator. Depending on the rotational speed of the rotating pressure patterns, these spinning modes can be divided into attenuating modes and propagating modes, and it is the propagating modes that are transmitted through the acoustically lined duct and radiate objectionable noise to the outside. Although sound suppressing treatments are often used, usually extending over the entire inner surface of the fan duct, such treatments are employed generally and are not tailored for absorption of spinning mode sound. Non-uniform circumferential acoustical treatment is disclosed in several patent applications and patents assigned to the same assignee, but is provided for a much different reason. By way of example, these include application Ser. No. 442,893 filed on Feb. 15, 1974 by Norman J. Lipstein, entitled "Acoustic Duct with Asymmetrical Acoustical Treatment" now U.S. Pat. No. 3,890,060; and U.S. Pat. No. 3,508,838 granted on Apr. 28, 1970 to A. J. Martenson. The present invention is directed to an improved acoustic duct for the more effective suppression of sounds generated by a jet engine fan or other source of spinning mode pressure fields.